The present invention generally relates to mobile telecommunications systems, and more particularly to a system and method for determining whether to assign a macrocell communication frequency or a microcell communication frequency to a particular mobile communication terminal for a particular communication interval of a communication session.
Radiophone communication using mobile communication terminals in mobile telecommunication systems has become very popular. Generally, such mobile communication terminals include hand-held, personal communication terminals and vehicle mounted communication terminals (e.g., car phones). As a result of the increased popularity of radiophone communication, some geographic areas, particularly city areas, are becoming overloaded with requests for additional radiophone communication frequencies. However, the number of available radiophone communication frequencies is limited because the Federal Communication Commission (FCC) licenses only a finite number of radiophone communication frequencies for each geographic area. Thus, topologies for mobile telecommunication systems which maximize the use of a finite number of radiophone communication frequencies have been developed.
FIG. 1 illustrates a conventional topology for use with mobile telecommunication systems. The topology includes a relatively large geographic area called a macrocell area (typically from 30 to 200 square miles in area). The macrocell area is divided into a number of smaller geographical areas called microcell areas (each typically from 0.1 to 3 square miles in area). In the topology of FIG. 1, the macrocell area includes nine microcell areas.
Assigned to the macrocell area are a number of unique macrocell communication frequencies which are usable throughout the macrocell area. That is, only a single mobile communication terminal operating within the macrocell area may be assigned to any particular macrocell communication frequency. The communication frequencies assigned to a macrocell generally are not utilized in an adjoining macrocell in order to reduce interference problems.
Associated with the microcell areas are a number of microcell communication frequencies. Certain microcell areas share the same microcell communication frequencies. That is, multiple mobile communication terminals may be assigned to the same microcell communication frequency as long as all of the mobile communication terminals are located in different microcell areas which are geographically separated. In the case of the macrocell shown in FIG. 1 the same microcell communication frequencies could be used in microcells 1, 3 and 5. Because of geographic separation and with the use of the low power levels employed with most hand held mobile communication terminals, no interference would occur if the same communication frequencies are used in microcells 1, 3 and 5 thereby enabling more mobile communications terminals to be used within a particular macrocell without the need to preempt additional frequencies.
When a radiophone communication request from a mobile communication terminal is received by a mobile telecommunication system, the mobile telecommunication system must determine whether to assign to the particular mobile communication terminal a macrocell communication frequency or a microcell communication frequency. One manner in which conventional mobile telecommunication systems perform such an assignment is as follows. If the telecommunication system determines (based on information contained in the request from the mobile communication terminal) that the mobile communication terminal is a hand-held personal terminal, then the mobile telecommunication system assigns a microcell communication frequency to the mobile communication terminal. If, however, the mobile telecommunication system determines that the mobile communication terminal is a vehicle mounted communication terminal, then the mobile telecommunication system assigns a macrocell communication frequency to the mobile communication terminal.
One assignment procedure used by conventional mobile communication systems is based on the following assumptions. First, moving vehicle mounted communication terminals are likely to traverse multiple microcells during a particular communication session. If a microcell frequency were assigned to a vehicle mounted communication terminal, it is likely that multiple microcell frequencies would have to be used and multiple transfers or "handoffs" would be required as the vehicle moves though the microcells, tying up multiple frequencies and transfer resources for a single communication session. Therefore, a macrocell communication frequency which operates throughout the macrocell area is appropriate. Second, personal terminals are likely to travel only within a microcell area, if at all, during a particular communication session. Therefore, a microcell communication frequency having limited operational range is appropriate for a personal communication terminal.
This conventional assignment procedure is flawed, however, because personal terminals are often located in vehicles which traverse multiple microcells during a particular communication session. As a personal terminal in a vehicle travels from one microcell to another, the mobile telecommunication system must switch or handoff the personal terminal from one microcell to another, a process which is inherently inefficient and wasteful of communication resources.
Similarly, a vehicle-mounted terminal may spend long periods of conversation time within a microcell area because it is parked, or delayed by traffic conditions. It is inefficient and wasteful to utilize a macrocell communication frequency under such conditions.
Multiple handoffs place a great strain on switching and control resources in mobile telecommunication systems. In fact, multiple handoffs may impede the reliable performance of small microcell systems. That is, customers who select a microcell based service may experience dropped calls (that is, disconnections) when they attempt to use their personal terminals in moving vehicles if the handoff procedures are not properly executed.
Therefore, an improved system and method for determining whether to assign a macrocell communication frequency or a microcell communication frequency to a mobile communication terminal is required, particularly a personal communication terminal which is being used in a moving vehicle, or a vehicle-mounted terminal which is being used in a stationary vehicle such that switching and control resources and communications frequencies in mobile telecommunication systems are conserved.